Aldehyde oxidase is coamplified with the World's most common Culex mosquito insecticide resistance-associated esterases

The evolution and spread of insecticide resistance is an important factor in human disease prevention and crop protection. The mosquito Culex quinquefasciatus is the main vector of the disease filariasis and a member of a species complex which is a common biting nuisance worldwide. The common insecticide resistance mechanism in this species involves germline amplification of the esterases est alpha 2(1) and est beta 2(1). This amplification has arisen once and rapidly spread worldwide. Less common and more variable resistance phenotypes involve coamplification of est alpha 3 and est beta 1, or individual amplification of a single est beta 1, different alleles of the same est alpha and est beta gene loci. Est alpha 2(1) and est beta 2(1) are on the same large fragment of amplified DNA (amplicon) 2.7 kb apart. We have now shown that this amplicon contains another full-length gene immediately 5' of est alpha 2(1) which codes for a molybdenum-containing hydroxylase, with highest homology to aldehyde oxidase (AO) from other organisms. The full-length putative AO gene is not present on the est alpha 3/est beta 1 or est beta 1 amplicons, but multiple truncated 5' ends of this gene are present around the presumed est alpha 3/est beta 1 amplicon breakpoint. Polymerase chain reaction (PCR) analysis of insecticide-susceptible genomic DNA demonstrated that a different allele of the putative AO gene in its non-amplified form is immediately 5' of est alpha. The 'AO' gene on the est alpha 2(1)/est beta 2(1) amplicon is expressed and resistant insects have greater AO activity. This AO activity is sensitive to inhibition by an aldehyde-containing herbicide and pesticide. This enzyme may confer a selective advantage to these insects in the presence of insecticide, as AO in mammals is believed to be important in the detoxification process of several environmental pollutants.